1. Technical Field
The present invention relates to a method for producing a catalyst-layer-supporting substrate, a method for producing a membrane-electrode assembly and a method for producing a fuel cell.
2. Related Art
Polymer electrolyte fuel cells, in particular, methanol-typed polymer electrolyte full cells which use a methanol solution as a fuel can operate at low temperatures and are small in size and light in weight. Therefore, applications of them to a power supply for mobile devices have been researched and studied in recent years. The performance of conventional fuel cells has, however, not reached a level which enables a wide propagation thereof. Since fuel cells change chemical energy to electric power through an electrocatalysis reaction, highly active catalysts are vital to the development of high-performance fuel cells.
PtRu is generally used as an anode catalyst of a fuel cell. A voltage loss by the PtRu catalyst is about 0.3 V relative to a theoretical voltage of 1.2 V which is obtained by the electrocatalysis reaction, and there exists a need for obtaining a highly active (methanol oxidation activated) anode catalyst which surpasses PtRu. There have been carried out various studies on improvement in methanol oxidation activity which includes the addition of other elements to PtRu, and reports thereof are available.
As a conventional catalyst synthesis method, the solution methods have been generally adopted in which solutions of basic salts are used to synthesize a catalyst through impregnation, precipitation, and liquid-phase reduction. With the solution methods, however, there exists an inherent problem that the control of a surface of a catalyst is difficult with respect to elements which are difficult to be reduced and elements which are difficult to be alloyed.
On the other hand, a catalyst synthesis using the sputtering method or deposition method is advantageous in the aspect of controlling materials, and the inventor et al have found out a highly active catalyst through the sputtering or deposition process. When preparing a catalyst using the conventional sputtering method or deposition method, however, there still exists a problem of controlling the catalyst layer pore structure, and hence, it is desired to improve the preparation process and increase further the properties of fuel cells and the utilization efficiency of noble metals. For example, in an anode electrode, the utilization efficiency of a catalyst largely depends on the density of a three-phase interface in fuel/catalyst/proton conductive material. Due to this, in order to obtain sufficient fuel cell properties, it is necessary to control the pore structure of a catalyst layer and increase the density of the three-phase interface.
There have been disclosed some catalyst layer forming methods using the sputtering process until now. For example, it is reported in JP-A 2001-307751 (KOKAI) that catalyst layers and carbon are sputtered simultaneously or sequentially on to an electrolytic membrane. It is reported in WO 2002/073722 that a laminated structure made up of carbon powder and a catalyst is formed on an electrode substrate by a method including a vapor phase deposition process. It is reported in JP-A 2004-281177 (KOKAI) that a two-layer catalyst structure is formed by sputtering Si before a catalyst is sputtered, and this process is said to be effective in suppressing the growth of particles of a catalyst. However, it cannot be said that these processes are good enough to improve the catalyst layer pore structure, and hence, there is a demand for development of new processes.